Call coverage arrangements allow calls to a principal's telephone station to be answered by someone other than the principal for the purpose of receiving messages, etc. In the past, such arrangements typically consisted of a subscription to a call answering service or a bridged appearance of a principal's line at a secretary's station. More recently, other options have become available. For instance, the DIMENSION.RTM. Private Branch Exchange System (DIMENSION is a registered trademark of American Telephone and Telegraph, Inc.), manufactured by Western Electric, Inc., offers two features called Call Forwarding--All Calls and Call Forwarding--Busy/Don't Answer. For both of these features, an incoming call is automatically forwarded to a telephone number specified by the principal at the time the feature is activated. The forwarding occurs immediately for the first mentioned feature, whereas for the latter feature forwarding occurs only if the principal's line is busy or if a call remains unanswered at the principal's station for a specified period of time. In addition, the DIMENSION PBX System offers a call pickup feature in which a call to a given station can be answered by another station in a call pickup group by dialing a special access code from the answering station.
The HORIZON Communications System (HORIZON is a registered trademark of American Telephone and Telegraph, Inc.), manufactured by Western Electric, Inc., extends the coverage concept further by allowing principals to prespecify in memory on a semipermanent basis one or more coverage stations to which calls to the principal station are simultaneously directed. The coverage feature of the HORIZON System operates as follows. Alerting of an incoming call at a principal station is performed by conventional ringing and flashing. Simultaneously, each of the stations of a prespecified coverage group is also flashed on a dedicated "coverage" button. The call may be answered at any of these stations. Presumably, the covering users at each station of the coverage group know their respective order of preference for covering calls to the principal station. Thus, after a certain period of time of principal nonanswer the first preferred covering user, if available, answers the call. After a longer time period of nonanswer, the second preferred covering user answers the call, and so on. Although this arrangement operates satisfactorily, it requires each covering user to observe and manually time, to some extent, the flashing of the coverage button. As an alternative, the system allows the principal station to be equipped with a "transfer ringing" option. Under this option, ringing is transferred sequentially to designated coverage group stations after specified periods of nonanswer. The flashing continues, however, at each of the coverage group stations until the call is answered or abandoned. In both the flashing and ring transfer arrangements, the distraction of each of the potential covering users throughout the ringing interval is undesirable.
Known telephone systems are also different in other areas of call coverage. For example, the known systems offer coverage only in principal "busy" or "does not answer" situations. As another example, some systems offer a "send all calls" (SAC) feature. For example, in the HORIZON system, a principal may request that ringing be transferred to the designated coverage stations without ringing the principal station. When a SAC button is operated on the principal station, all future arriving calls are automatically rung at the designated coverage stations. No known system, however, offers a SAC feature that operates with respect to calls that are ringing at a principal station when the feature is activated. Thus, a principal who normally answers his calls but who, for any reason, does not wish to answer one or more arriving calls at a particular time, must either answer or be disturbed by ringing until a "don't answer" period expires, a secretary answers or the calls are abandoned, depending on the coverage applicable to the principal.